Using CFD to Assess Low Frequency Damping

During offshore installation operations, floating bodies are often moored using soft mooring which are designed to withstand the environmental forces. Large amplitude motions often occur due to excitation by slowly varying wind and wave drift forces. To analyze these motions the dynamic system has to be accurately described, which includes an estimation of the added mass and damping coefficients. In general, the added mass can be accurately calculated with traditional potential theory. However for the damping this method is not adequate because viscous effects play an important role. Generally these data are obtained using model tests. This paper validates the CFD methodology as an alternative to model tests to evaluate the viscous damping. The aim is to define a standard procedure to derive viscous damping coefficients for surge, sway and yaw motion of floating bodies. To estimate viscous damping in CFD, a 3D model of the launch and float-over barge H-851 was used. For this barge, model test data is available which could be compared with the results of the CFD analysis. For the simulations, the commercial package STAR-CCM+ with the implicit unsteady solver for Reynolds-Averaged Navier-Stokes (RANS) equations was used. The turbulence model implemented was the k-Omega-SST. Numerical errors have been assessed performing sensitivity analysis on time step and grid size. Damping has been investigated by performing decay simulations as in the model tests, taking the effect of coupling among all motions into account. The P-Q fitting method has been used to determine the linear and quadratic component of the damping. Numerical results are validated with those obtained from the towing tank. Results show that CFD is an adequate tool to estimate the low frequency damping in terms of equivalent damping. More investigations are required to determine the linear and quadratic component.